The present invention relates to an improved regenerable sorbent, method and system for absorbing carbon dioxide (CO2) from a habitable environment. More particularly, this invention relates to a CO2 sorbent which performs equally well in a high or in a low humidity environment.
Exposure to carbon dioxide partial pressures which exceed about 7.6 mm Hg (millimeters of mercury, partial pressure of about 1%), for extended periods of time are known to cause health problems in human beings and in other mammals. As a result, in enclosed environs such as those existing in submarines, spacecrafts or space suits, carbon dioxide partial pressures are typically maintained below about 1% through the use of solid carbon dioxide sorbents such as soda lime, zeolite and carbon molecular sieves, solid oxides, alkali metal carbonates, alkali metal hydroxides, amines, and combinations of the aforesaid sorbents.
The aforesaid CO2 sorbent methodologies are sensitive to the relative humidity of the environs in which they are being utilized, and generally will operate at a higher efficiency with higher relative humidity, and with lower efficiency at a lower relative humidity. Conventional CO2 sorbents require an atmosphere dew point, exceeding about 2xc2x0 C. It would be highly desirable to provide a CO2 sorbent and a method for absorbing CO2 from environs, in which the sorbent is not sensitive to the relative humidity of the environs being desorbed.
This invention relates to a carbon dioxide sorption system and method which increases the regenerable CO2 sorption capacity of the sorbent by more than twice that of the above-noted prior art CO2 sorbents. The regenerable CO2 sorbent used in the system and method of this invention is also not sensitive to the relative humidity in the closed environment atmosphere. The sorbent is a regenerable high capacity amine sorbent which continuously removes CO2 from the atmosphere in enclosed environments. The sorbent is composed of a molecular entity which contains predominantly secondary amines and one or more nitrile functional groups. A preferred sorbent is a reaction product of tetraethylenepentamine (TEPA) and acrylonitrile (AN). AN has the following molecular structure: H2Cxe2x95x90CHCxe2x95x90N. The reaction of TEPA with AN substantially converts TEPA""s primary amines to secondary amines. This conversion enhances the regenerable cyclic CO2 sorption capacities by more than 2.5 times, and significantly reduces amine volatility to essentially immeasurable levels. One reaction product of TEPA and AN, which we refer to herein as TEPAN, is a reaction product resulting from the combination of 1.0 mole TEPA and 3 moles AN, wherein a branched acyclic form of TEPAN shown below is formed. 
Another reaction product of TEPA and AN, is a compound resulting from the combination of 1.0 mole TEPA and 2 moles AN, wherein a non-branched acyclic form (shown below) is formed. 
The reaction of TEPA with 1 to approximately 3 moles and AN which forms either branched or straight chain cyclic and non-cyclic reaction products which are generally referred to as TEPAN. It is noted that TEPA includes several acyclic and cyclic amine components wherein overall there are approximately 40% primary, 45% secondary, and 15% tertiary amines. The acyclic and cyclic amine groups in TEPA are approximately equal in content. It has been determined that when one mole of TEPA and 2.3 moles of AN are reacted, the AN reactant reacts preferentially with the primary amines in TEPA so as to substantially increase the ratio of secondary amines to primary amines from a ratio of approximately 1/1 to a ratio of approximately 10/1. Thus, the resultant combination of TEPAN molecules contains approximately 8% primary amine groups, 78% secondary amine groups, and 14% tertiary amine groups. Thus, the TEPAN formed from 1 mole of TEPA and 2.3 moles of AN is comprised predominantly of secondary amine functional groups and nitrile functional groups in a ratio of about 1.5/2.0 (amine/nitrile).
The TEPAN sorbent is applied to a polymeric support structure. The polymeric support structures including acrylic esters are particularly desirable. The stability of the aforesaid amine sorbent enables its long term use in removing CO2 from enclosed environments. We have demonstrated that the TEPAN sorbents of this invention are capable of removing CO2 for periods approaching one year in duration without significant degradation and loss of performance. Sorbent beds formed in accordance with this invention exhibit essentially no release of amines or their degradation products during use. The ability of the sorbent bed to operate equally efficiently in both high and low humidity conditions is unique.
It is therefore an object of this invention to provide a carbon dioxide (CO2) sorbent system and method which is highly stable and has a long operating life.
It is an additional object of this invention to provide a system and method of the character described which is equally efficient in absorbing CO2 in either a high humidity or a low humidity environment.
It is a further object of this invention to provide a system and method of the character described wherein the sorbent can be regenerated in situ.
It is another object of this invention to provide a system and method of the character described wherein the sorbent is a molecular entity which is composed of predominantly secondary amines with one or more nitrile functional groups.